Electrosynthesis is a method for production of chemical reaction(s) that is electrically driven by passage of an electric current, typically a direct current (DC), through an electrolyte between an anode electrode and a cathode electrode. An electrochemical cell is used for electrochemical reactions and comprises anode and cathode electrodes immersed in an electrolyte with the current passed between the electrodes from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, the DC is passed between the two electrodes in an aqueous electrolyte to split water, the reactant, into component product gases, namely, hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
Water electrolysers have typically relied on pressure control systems to control the pressure between the two halves of an electrolysis cell to insure that the two gases, namely, oxygen and hydrogen produced in the electrolytic reaction are kept separate and do not mix.
In the conventional mono-polar cell design presently in wide commercial use today, one cell or one array of (parallel) cells is contained within one functional electrolyser, or cell compartment, or individual tank. Therefore, each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent No. 302,737, issued to A. T. Stuart (1930). The current is taken from a portion of a cathode in one cell to the anode of an adjacent cell using point-to-point electrical connections using the above-mentioned bus bar assembly between the cell compartments. The current is usually taken off one electrode at several points and the connection made to the next electrode at several points by means of bolting, welding or similar types of connections and each connection must be able to pass significant current densities.
Most filter press type electrolysers insulate the anodic and cathodic parts of the cell using a variety of materials that may include metals, plastics, rubbers, ceramics and various fibre based structures. In many cases, O-ring grooves are machined into frames or frames are moulded to allow O-rings to be inserted. Typically, at least two different materials form the assembly necessary to enclose the electrodes in the cell and create channels for electrolyte circulation, reactant feed and product removal.
WO98/29912, published Jul. 9, 1998, in the name The Electrolyser Corporation Ltd. and Stuart Energy Systems Inc., describes such an electrolyser system configured in either a series flow of current, single stack electrolyser (SSE) or in a parallel flow of current in a multiple stack electrolyser (MSE). Aforesaid WO98/29912 provides details of the components and assembly designs for both SSE and MSE electrolysers.
As used herein, the term "cell" or "electrochemical cell" refers to a structure comprising at least one pair of electrodes including an anode and a cathode with each being suitably supported within a cell stack configuration. The latter further comprises a series of components such as circulation frames/gaskets through which electrolyte is circulated and product is disengaged. The cell includes a separator assembly having appropriate means for sealing and mechanically supporting the separator within the enclosure and an end wall used to separate adjacent "cells". Multiple cells may be connected either in series or in parallel to form cell stacks and there is no limit on how many cells may be used to form a stack. In a stack the cells are connected in the same way, either in parallel or in series. A cell block is a unit that comprises one or more cell stacks and multiple cell blocks are connected together by an external bus bar. A functional electrolyser comprises one or more cells that are connected together either in parallel, in series, or a combination of both as detailed in PCT application WO98/29912.
Depending on the configuration of such a cell stack electrochemical system, each includes an end box at both ends of each stack in the simplest series configuration or a collection of end boxes attached at the end of each cell block. Alternative embodiments of an electrolyser includes end boxes adapted to be coupled to a horizontal header box when both a parallel and series combination of cells are assembled.
In the operation of the cell stack during electrolysis of the electrolyte, the anode serves to generate oxygen gas whereas the cathode serves to generate hydrogen gas. The two gases are kept separate and distinct by a low permeable membrane/separator. The flow of gases and electrolytes are conducted via circulation frames/gasket assemblies which also act to seal one cell component to a second and to contain the electrolyte in a cell stack configuration in analogy to a tank.
The rigid end boxes can serve several finctions including providing a return channel for electrolyte flowing out from the top of the cell in addition to serving as a gas/liquid separation device. They may also provide a location for components used for controlling the electrolyte level, i.e. liquid level sensors and temperature, i.e. for example heaters, coolers or heat exchangers. In addition, with appropriate sensors in the end boxes individual cell stack electrolyte and gas purity may be monitored. Also, while most of the electrolyte is recirculated through the electrolyser, an electrolyte stream may be taken from each end box to provide external level control, electrolyte density, temperature, cell pressure and gas purity control and monitoring. This stream would be returned to either the same end box or mixed with other similar streams and returned to the end boxes. Alternatively, probes may be inserted into the end boxes to control these parameters.
Thus, prior art cells generally comprise a plurality of planar members comprising metallic current carriers, separators, gaskets and circulation frames suitably functionally ordered, and arranged adjacently one to another in gas and electrolyte solution sealed engagement with and between the end walls of the cell(s). The non-metallic components such as the gaskets, separators and circulation frames are generally formed of compressible elastomeric materials. Assembly of the cell by compression of the cell components together provides, generally, satisfactory fluid tight seals within the cell block. In prior art cells, such as the MSE described in aforesaid WO98/29912 the cells are arranged in electrical parallel connection within each stack and series connection across adjacent stacks, wherein the metal current carriers which include the electrode members, per se, extend to the top, bottom and side edges of the cell, as do the nonmetallic components, such that the peripheries of the elastomeric and metallic planar members are coplanar.
A preferred embodiment described in aforesaid WO98/29912 comprises a four stack, two-cell double electrode plate (DEP) arrangement wherein the DEP(s) traverse two adjacent cells of a pair of stacks. Apart from the header gasket and the DEP(s), no other planar member of the aforesaid cell block extends beyond a single cell stack. Although the aforesaid monopolar electrolyser is of a compact cell design having relatively low intercell resistance factors, it is still highly desirable to further enhance the compactness and reduce the footprint and area occupied by the cell block.